feat!: scope chat model override settings to organizations (#28442)

Moves chat model override settings from deployment scope to organization
scope, following the org-scoping of chat model configs in #28440.

Depends on #28440 (based on
`ethan/chat-models-availability-consolidation`).

Remote dogfood UAT passed on this change, including multi-organization
scenarios on a licensed deployment with real models.

## Problem

Chat model configs are now owned by organizations, but the override
settings that reference them stayed deployment-wide: admin
subagent/title/compaction overrides in `site_configs`, personal
overrides in `user_configs`, and the advisor model embedded in the
advisor runtime JSON. A single deployment-level override can only point
at one organization's model config, so overrides effectively worked for
the default organization only, and title generation had to soften its
hard-failure contract to tolerate unusable overrides.

## Fix

Admin overrides (general, explore, title generation, compaction,
advisor) and personal overrides (root, general, explore) become
organization-scoped rows in two new typed tables,
`chat_organization_model_overrides` and `chat_user_model_overrides`.
Composite foreign keys on `(organization_id, model_config_id)` make
cross-organization model references unrepresentable. The migration
deletes the legacy serialized `site_configs`/`user_configs` override
keys instead of migrating them; admins and users re-select models in the
new organization-scoped settings.

New endpoints live under
`/api/experimental/organizations/{organization}/chats/model-overrides`
(admin defaults) and
`/api/experimental/organizations/{organization}/members/{user}/chats/model-overrides`
(personal overrides); the legacy deployment-level routes are removed.
chatd resolves overrides via the chat's organization, restoring the
hard-failure contract for configured-but-unusable title generation
overrides. The deployment-level `AllowUsers` switch and advisor runtime
limits stay deployment-scoped.

The Coder Agents settings UI gains a per-organization "Defaults &
overrides" page under AI Settings > Models, and the personal overrides
page gains an organization selector for users in multiple organizations.
Advisor telemetry now reports per-organization override rows.

**BREAKING CHANGE**: Existing deployment-level overrides are dropped,
not migrated. Every organization starts with no overrides after upgrade,
and model selections must be re-applied in the new settings. The legacy
deployment-level override endpoints are removed.

Closes https://linear.app/coder/issue/CODAGT-872

> [!NOTE]
> Xum (AI agent) authored this PR on behalf of @ibetitsmike.

<!-- xum-attribution: model=claude-opus-4-6 thinking=high -->

---------

Co-authored-by: Ethan Dickson <ethan@coder.com>

coderd/database/querier_test.go

@@ -6491,6 +6491,85 @@ func TestGetUserStatusCounts(t *testing.T) {
 	}
 }
 
+func TestGetChatOrganizationModelOverridesByContext(t *testing.T) {
+	t.Parallel()
+	db, _ := dbtestutil.NewDB(t)
+	ctx := testutil.Context(t, testutil.WaitShort)
+
+	availableOrg := dbgen.Organization(t, db, database.Organization{})
+	availableModel := dbgen.ChatModelConfig(t, db, database.ChatModelConfig{
+		Model:          "bulk-available-" + uuid.NewString(),
+		Enabled:        true,
+		OrganizationID: availableOrg.ID,
+	})
+	require.NoError(t, db.UpsertChatOrganizationModelOverride(ctx, database.UpsertChatOrganizationModelOverrideParams{
+		OrganizationID: availableOrg.ID,
+		Context:        "general",
+		ModelConfigID:  availableModel.ID,
+	}))
+
+	// An override referencing a disabled model reports the model unavailable.
+	// dbgen coerces Enabled=false to true, so disable it after insertion.
+	disabledOrg := dbgen.Organization(t, db, database.Organization{})
+	disabledModel := dbgen.ChatModelConfig(t, db, database.ChatModelConfig{
+		Model:          "bulk-disabled-" + uuid.NewString(),
+		OrganizationID: disabledOrg.ID,
+	})
+	_, err := db.UpdateChatModelConfig(ctx, database.UpdateChatModelConfigParams{
+		ID:                   disabledModel.ID,
+		Model:                disabledModel.Model,
+		DisplayName:          disabledModel.DisplayName,
+		Enabled:              false,
+		IsDefault:            disabledModel.IsDefault,
+		ContextLimit:         disabledModel.ContextLimit,
+		CompressionThreshold: disabledModel.CompressionThreshold,
+		Options:              disabledModel.Options,
+		AIProviderID:         disabledModel.AIProviderID,
+	})
+	require.NoError(t, err)
+	require.NoError(t, db.UpsertChatOrganizationModelOverride(ctx, database.UpsertChatOrganizationModelOverrideParams{
+		OrganizationID: disabledOrg.ID,
+		Context:        "general",
+		ModelConfigID:  disabledModel.ID,
+	}))
+
+	// Deleted organizations are excluded from the bulk read.
+	deletedOrg := dbgen.Organization(t, db, database.Organization{})
+	deletedOrgModel := dbgen.ChatModelConfig(t, db, database.ChatModelConfig{
+		Model:          "bulk-deleted-org-" + uuid.NewString(),
+		Enabled:        true,
+		OrganizationID: deletedOrg.ID,
+	})
+	require.NoError(t, db.UpsertChatOrganizationModelOverride(ctx, database.UpsertChatOrganizationModelOverrideParams{
+		OrganizationID: deletedOrg.ID,
+		Context:        "general",
+		ModelConfigID:  deletedOrgModel.ID,
+	}))
+	require.NoError(t, db.UpdateOrganizationDeletedByID(ctx, database.UpdateOrganizationDeletedByIDParams{
+		ID:        deletedOrg.ID,
+		UpdatedAt: dbtime.Now(),
+	}))
+
+	rows, err := db.GetChatOrganizationModelOverridesByContext(ctx, "general")
+	require.NoError(t, err)
+	require.Len(t, rows, 2)
+	byOrg := make(map[uuid.UUID]database.GetChatOrganizationModelOverridesByContextRow, len(rows))
+	for _, row := range rows {
+		byOrg[row.OrganizationID] = row
+	}
+	available := byOrg[availableOrg.ID]
+	require.True(t, available.ModelAvailable)
+	require.Equal(t, availableModel.Model, available.Model)
+	require.NotEmpty(t, available.ProviderType)
+	unavailable := byOrg[disabledOrg.ID]
+	require.False(t, unavailable.ModelAvailable)
+	require.Empty(t, unavailable.Model)
+
+	otherContext, err := db.GetChatOrganizationModelOverridesByContext(ctx, "advisor")
+	require.NoError(t, err)
+	require.Empty(t, otherContext)
+}
+
 func TestOrganizationDeleteTrigger(t *testing.T) {
 	t.Parallel()